Recently, as electronic appliances have been downsized and lightened, and the use of portable electronic appliances has been generalized, many active studies have been conducted to develop lithium secondary batteries having high energy density.
A lithium secondary battery comprises a cathode and an anode, each formed of a material capable of lithium ion intercalation/deintercalation. An organic electrolyte or polymer electrolyte is injected between the cathode and the anode. Such lithium secondary batteries generate electric energy via redox reactions occurring upon lithium ion intercalation/deintercalation at the cathode and the anode.
However, such lithium secondary batteries have problems related with their safety, such as ignition and explosion, caused by the use of a non-aqueous electrolyte. The above problems become more serious as the capacity density of a battery increases. Particularly, when a battery is overcharged to a voltage higher than the normal drive voltage, an excessive amount of lithium is deintercalated from the cathode and the lithium produces dendrite at the anode so that both the cathode and the anode become thermally unstable, resulting in a rapid exothermic reaction including decomposition of the electrolyte. Due to the exothermic reaction, the battery causes a thermal runaway phenomenon followed by ignition and explosion, and shows a serious problem related with its safety.
Many attempts have been made to inhibit such ignition or explosion of a battery caused by overcharge or an increase in the temperature inside the battery. For example, an additive for a non-aqueous electrolyte has been used. However, it is necessary to introduce a great amount of the additive in order to improve the safety of a battery by using the additive for a non-aqueous electrolyte. Such direct introduction of the additive may cause degradation of the quality of the battery.
Therefore, there is a need for developing a novel means to improve the safety of an electrochemical device including a battery.